SE 453,017 discloses an optical analysing equipment which may be used in e.g. a chimney for determining different parameters of gaseous substances, such as SO.sub.2, NO.sub.2 and NO, in the gas volume flowing through the chimney. The parameters are determined by emitting light from a light transmitter On one side of the chimney to the gas volume, by receiving the light, which is transmitted through the gas volume, by a light receiver on the other side of the chimney, and by analysing the received light by means of the actual analysing equipment which may be positioned at a distance from the chimney.
Before mounting the light transmitter and the light receiver in the chimney, the analysing equipment is calibrated. It should then be calibrated at regular intervals to compensate for, inter alia, the ageing of the components. In order to carry out the calibration, it is today necessary to dismount the light transmitter and the light receiver from the chimney and instead mount them on a container to which different gases of a known composition and concentration may be supplied. This is a time-consuming method of carrying out the calibration, which also demands a great deal of work and which is not suitable to use if frequent calibration of the equipment is desirable.
U.S. Pat. No. 4,381,153 discloses an opacity monitor which permits calibration on site. The opacity monitor comprises a light source mounted on one side of a duct, and a detector mounted on the opposite side of the duct. The light source and the detector are rotatable by motor means between a first position aligned with each other along an open light path defined across the duct, and a second position aligned with each other on opposite sides of a calibration tube extending across the duct. Shutters rotate with the light source and the detector, and the shutters are aligned with the calibration tube when the light source and the detector are aligned with the open light path, and vice versa. By rotating the light source and the detector to the second position, calibration can thus be carried out when the device is mounted at the duct.
However, it is a well-known fact that movable parts often cause reliability and maintenance problems. Moreover, problems may arise when using the device according to U.S. Pat. No. 4,381,153 in very large chimneys in which it may be difficult to mount the calibration tube.
One object of the invention therefore is to provide an optical analysing equipment which permits calibration on site, with as few movable parts as possible.
A further object of the invention is to provide an optical analysing equipment which is intended for ducts having an extremely great diameter and which permits calibration on site.